Are there alternate methods to activate Steam CD Keys?

Just recently (today), I bought a Humble Bundle and I do not have access to a PC with the Steam Client...

Is there any alternate ways to activate the keys I've got from my bundle? Like calling APIs manually or external programs? (Re-downloading and reinstalling Steam is NOT an option)

Or is it not possible whatsoever?

I remembered the Humble Bundle had automatic key redemption... But it's not there anymore as the key redemption API has been shut off.

Aside from the client, you can also activate Steam keys through the Steam store website.

https://store.steampowered.com/account/registerkey

At the time of writing, there's no way to get to this page without the direct link, so I suggest bookmarking it.

You can also pre-fill the key like this:

https://store.steampowered.com/account/registerkey?key=AAAAA-BBBBB-CCCCC

I just looked it up. I ended up on this article.

Important: Ensure that you are activating your game through the Steam application. The Steam website cannot be used to activate a game. If you have not yet done so, you can download and install the Steam application by clicking on the green "Install Steam Now" button here: http://store.steampowered.com/about/

So no, it's not possible to activate games without using the client.